The purpose of the proposed study is to measure renal tubular pCO2 directly, using a newly developed micro-pCO2 electrode, and to utilize this new information to reassess the mechanisms regulating bicarbonate reabsorption. The experiments will be performed in rats using micropuncture methodology. The specific research objectives are as follows: 1. Characterization of the pattern pCO2 in the proximal and distal tubule, and its relation to efferent arteriolar, renal venous and arterial pCO2. 2. Utilization of in situ tubular pH and pCO2 measurements to reassess the evidence favoring hydrogen ion secretion as the primary mechanism for bicarbonate reabsorption. Is a true disequilibrium pH (in situ pH more acid than that calculated at equilibrium) present in the tubule? 3. Characterization of the influence of variations in the rate of bicarbonate reabsorption and hydrogen ion secretion (induced by acid base disorders) on tubular pH and pCO2.